This document provides an overview of health systems strengthening. It defines key concepts including health systems, health system strengthening, and the four main functions of a health system: stewardship, financing, human and physical resources, and service delivery. It then discusses each function in more detail, including how policies and programs can influence health outcomes through strengthening different parts of the health system. The goal is to help organizations and implementers understand health systems and how their work can benefit from health systems approaches.